Topic 2.1 Introduction to Biodiversity: describe the three levels of biodiversity and explain how genetic and species diversity contribute to ecosystem resilience.

What this topic is asking

The College Board (Topic 2.1) wants you to describe the three levels of biodiversity, distinguish species richness from species evenness, and explain why genetic diversity matters for survival. The central idea is that greater diversity makes populations and ecosystems more resilient, that is, better able to withstand and recover from disturbance.

The three levels of biodiversity

Each level matters: genetic diversity lets populations adapt, species diversity supports ecosystem functions, and habitat diversity provides the range of environments that different species need.

Species richness and evenness

So two communities with the same richness can differ in diversity if one is dominated by a single abundant species (low evenness).

Why genetic diversity matters

Genetic diversity is the raw material for adaptation. A population with many different alleles is more likely to contain individuals whose traits let them survive a new stress, such as a disease, drought or temperature change. Those survivors reproduce and pass on the helpful alleles, so the population adapts over generations. A population with low genetic diversity has fewer options and may be unable to adapt fast enough.

Two processes reduce genetic diversity. A population bottleneck is a sharp reduction in population size (from a disaster, overhunting or habitat loss) that leaves only a small, genetically limited group of survivors. Genetic drift, the random change in allele frequencies, has a larger effect in small populations and can eliminate alleles by chance. Both leave populations less able to adapt and more vulnerable to inbreeding and future stress.

Biodiversity and resilience

The reason biodiversity matters at the ecosystem scale is resilience: the ability of an ecosystem to resist disturbance and recover from it. A diverse ecosystem usually has more species able to perform each function, so if one species is lost, another can take its place and the ecosystem keeps working. Diverse ecosystems are therefore more stable and more able to provide the ecosystem services that Topic 2.2 covers. This is why biodiversity loss is so serious: it removes the redundancy that lets ecosystems absorb shocks. Topic 2.1 sets up the rest of Unit 2 by establishing what biodiversity is and why it is valuable, before later topics examine how it is generated (island biogeography, adaptations, succession) and how it is lost.

Try this

Q1. Identify the level of biodiversity that refers to the variety of alleles within a population. [1 point]

• Cue. Genetic diversity.

Q2. Explain why a population with low genetic diversity is more vulnerable to a new disease. [2 points]

• Cue. With few different alleles, fewer individuals are likely to carry resistance, so a smaller fraction survives and the population may be unable to adapt.